Grand Prismatic Spring is the largest hot spring in the United States and third largest in the world at 370 feet in diameter. Its stunning colors range from deep blue in the center to green, yellow, orange, and red on the edges. The colors are produced by heat-loving bacteria and algae at different temperature zones. Center temperature reaches 160°F. A boardwalk provides close-up viewing, while the Fairy Falls Trail overlook offers panoramic views from above.